Communication pubs.acs.org/JACS
Metal−Organic Framework Nodes Support Single-Site Magnesium− Alkyl Catalysts for Hydroboration and Hydroamination Reactions Kuntal Manna,† Pengfei Ji,† Francis X. Greene,† and Wenbin Lin* Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States S Supporting Information *
tionation reactions.20 Herein, we report a simple strategy to stabilize active heteroleptic Mg-alkyl species at secondary building unit (SBUs) of TPHN-MOF (TPHN = 4,4′-bis(carboxyphenyl)-2-nitro-1,1′-biphenyl) to afford highly active and reusable single-site solid catalysts for hydroboration of carbonyl and imine compounds and for hydroamination of aminoalkenes. Site isolation within the MOF stabilizes catalytically active heteroleptic Mg-alkyl species by shutting down Schlenk-type intermolecular ligand redistribution reactions (Figure 1). TPHN-MOF of UiO-69 topology was synthesized via a solvothermal reaction between ZrCl4 and H2TPHN in the presence of DMF and trifluoroacetic acid in 95% yield.14d The metalation of Zr3(μ3-OH) sites in SBUs of TPHN-MOF was performed by treating TPHN-MOF with Me2Mg in THF at room temperature to afford Mg-functionalized MOF material (TPHN-MOF-MgMe) as a yellow solid (Figure 2a). During the metalation reaction, an equivalent amount of methane was generated, which was identified and quantified by GC analysis (Figure S4, Supporting Information [SI]). The disappearance of the νμ3‑OH band (∼3629 cm−1, KBr) in the infrared spectrum of MOF-MgMe indicated that the metalation occurred at Zr3(μ3OH) sites (Figure S5, SI). Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the digested MOF-MgMe revealed 100% metalation at the Zr3(μ3-OH) sites, corresponding to four Mg centers per Zr6 node. Crystallinity of TPHNMOF was maintained upon metalation, as shown by the similarities in the PXRD patterns of TPHN-MOF and MOFMgMe (Figure 2b). SEM images showed that TPHN-MOF has particle sizes of ∼2−3 μm (Figure S9, SI). The single-crystal Xray diffraction study revealed that MOF-MgMe crystallizes in the Fm3̅m space group, with the Zr6(μ3-O)4(μ3-OH)4 SBUs connected by the TPHN bridging linkers to afford the 12connected fcu topology. However, due to crystallographic disorder of the MgMe moiety, the Mg coordination environments in MOF-MgMe could not be established by X-ray crystallography (Figure S10, SI). The four MgMe moieties are randomly distributed among the eight μ3-oxo positions. In addition, every μ4-O-MgMe moiety is in close proximity to six carboxylate oxygen atoms; one or two of the carboxylate oxygen atoms could possibly tilt the Mg atom off the C3-axis through weak coordination. These disorders lead to low site occupancy of 1/6 for the MgMe moiety, if we assume Mg coordinates to two carboxylate groups, thus making Mg atoms impossible to observe in the Fm3̅m space group (Figure S10, SI).
ABSTRACT: Here we present the first example of a single-site main group catalyst stabilized by a metal− organic framework (MOF) for organic transformations. The straightforward metalation of the secondary building units of a Zr-MOF with Me2Mg affords a highly active and reusable solid catalyst for hydroboration of carbonyls and imines and for hydroamination of aminopentenes. Impressively, the Mg-functionalized MOF displayed very high turnover numbers of up to 8.4 × 104 for ketone hydroboration and could be reused more than 10 times. MOFs can thus be used to develop novel main group solid catalysts for sustainable chemical synthesis.
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espite their abundance and biocompatibility, alkaline earth metals such as Mg and Ca have found limited application in catalytic processes. Developing catalysts that use these metals can be challenging because of the propensity of heteroleptic complexes to undergo Schlenk-type and/or irreversible ligand redistribution reactions that result in catalytically incompetent homoleptic complexes.1 To counter such associative intermolecular ligand redistribution reactions, bulky chelating ligands, such as β-diketiminate,2 aminotroponates, aminotroponiminates,3 pybox,4 silylamido phenolate,5 and tris(oxazolinyl)borate,6 have been explored in the preparation of main group catalysts for a range of organic transformations, such as hydroamination,2c,3,7 hydrosilylation,8 hydroboration,9 hydrophosphination,10 Friedel−Crafts alkylation,4 and ring-opening polymerization.2a Although sterically encumbered ligands stabilize the heteroleptic species, they also often impede their catalytic activity by slowing down the binding and activation of substrates. In contrast, the immobilization of heteroleptic alkaline earth metal species in porous solid supports,11 such as metal−organic frameworks (MOFs), can provide site isolation without relying on bulky ligands and thus provide an alternative route for obtaining active catalysts. MOFs have recently emerged as an interesting class of highly porous molecular materials with many applications, including gas storage,12 separation,13 catalysis,14 nonlinear optics,15 biomedical imaging,16 chemical sensing,17 drug delivery,18 and solar energy harvesting.19 MOFs provide a highly tunable platform for the engineering of single-site solid catalysts for organic transformations that cannot be performed by traditional porous inorganic materials. We recently showed that highly active basemetal catalysts can be stabilized in bipyridyl-based MOFs via active site isolation, while analogous homogeneous catalysts undergo rapid intermolecular deactivation via ligand dispropor© XXXX American Chemical Society
Received: April 10, 2016
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DOI: 10.1021/jacs.6b03689 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Table 1. TPHN-MOF-Mg-Catalyzed Hydroboration of Ketones and Aldehydesa
Figure 1. Schematic showing the beneficial effect of active site isolation within a MOF on stabilizing heteroleptic Mg-species by preventing Schlenk-type ligand redistribution reactions.
a
Reaction conditions: 1.0 mg of MOF-MgMe, carbonyl substrate, hexanes, HBpin (1.1 equiv), 23 °C. bYield was determined by 1H NMR with mesitylene as the internal standard. Isolated yields are in parentheses. cTHF was used as the solvent.
Figure 2. (a) Synthesis of TPHN-MOF and metalation of its SBUs with Me2Mg to form MOF-MgMe. (b) Similarities among the PXRD patterns simulated from the CIF file (SI) of TPHN-MOF (black) and the PXRD patterns of MOF-MgMe (green), MOF-Mg samples recovered from hydroboration of acetophenone after run 1 (red) and after run 11 (blue), and after hydroboration of N-benzylidenebenzenamine show the retention of TPHN-MOF crystallinity after metalation and catalysis.
Mg afforded borate ester products from a range of carbonyl substrates, including alkyl, halogenated, and alkoxy-functionalized aryl ketones and aldehydes in essentially quantitative yields (Table 1). Pure hydroboration products were obtained by simply removing the solid catalyst via centrifugation followed by removal of the organic volatiles. Impressively, a turnover number (TON) of 84 000 was obtained for hydroboration of acetophenone (entry 2, Table 1). Linear and cyclic aliphatic ketones were efficiently reduced in excellent yields (entries 8− 10, Table 1). In addition, α,β-unsaturated carbonyl substrates such as cyclohexenone, trans-crotonophenone, and transcinnamaldehyde were reduced selectively at the carbonyl, leaving the CC bond intact (entries 10, 11, and 17, Table 1).
TPHN-MOF-Mg displayed excellent activity in the hydroboration of a wide range of carbonyl compounds with pinacolborane (Table 1). The hydroboration reactions were performed by treating ketones or aldehydes with equimolar HBpin in the presence of 0.1−0.01 mol % MOF-Mg in hexane or THF at room temperature. At a 0.05 mol % Mg loading, MOFB
DOI: 10.1021/jacs.6b03689 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Table 3. TPHN-MOF-Mg-Catalyzed Hydroamination of Aminoalkenesa
a
Reaction conditions: MOF-MgMe (1.0 mol % Mg), aminoalkene, benzene, 80 °C. bYields were determined by 1H NMR with mesitylene as the internal standard. Isolated yields are in parentheses.
experiments. ICP-MS analyses showed that the amounts of Mg and Zr leaching into the supernatant after the 1st run were only 1.6% and 0.02%, respectively, and after the 11th run were 1.3% and 0.02%, respectively. Moreover, no further hydroboration was observed after removal of MOF-Mg from the reaction mixture, which rules out the role of the leached Mg species in catalyzing hydroboration reactions (Figure S12, SI). An additional control experiment using the unmetalated TPHN-MOF did not afford the hydroboration product, indicating that the catalysis occurred at the supported Mg centers at the SBUs. Two types of pathways were proposed for the Mg-catalyzed hydroboration of carbonyls: (a) σ-bond metathesis involving insertion of a CO into a Mg−H bond9 or (b) zwitterionic mechanism without involvement of Mg−H bond species.21 The mechanism of MOF-Mg-catalyzed hydroboration reactions was investigated by spectroscopic techniques, gas quantification, and kinetic studies. The treatment of MOF-MgMe (1 mol % Mg) with acetophenone followed by addition of HBpin (PhCOMe:HBpin = 1.2:1.0) in benzene immediately generated MeBpin, and hydroboration reaction occurred without a detectable induction period. After completion of the hydroboration reaction, Zr3(μ4-O)-Mg(OCHMePh) was identified the intermediate (Figure S15, SI). In addition, Zr3(μ4-O)Mg(OMe), prepared by treatment of MOF-MgMe with 10 equiv (wrt Mg) of MeOH was also active in catalyzing hydroboration of acetophenone. We thus infer that the rapid reaction between Zr3(μ4-O)-MgMe with 1 equiv of HBpin afforded the Zr3(μ4-O)MgH intermediate, which is likely the active catalyst for hydroboration reactions. The empirical rate law, determined by the method of initial rates (
